Fait mathématique: 6668 is the number of trees on 21 vertices with diameter 5.
Culture générale: 6668 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: April 3rd is the day in 1936 that Bruno Richard Hauptmann is executed for the kidnapping and death of Charles Augustus Lindbergh, Jr., the baby son of pilot Charles Lindbergh.